1. Technical Field
The present invention relates to a transceiver device having a direct conversion structure and a calibration method used by the transceiver device.
2. Related Art
In the related art, wireless local area networks (WLANs) are an emerging application of wireless systems. Institute of Electrical and Electronics Engineers (IEEE) 802.11 is a WLAN standard. The 802.11b standard was the first to be commercialized, and the 802.11a standard was developed to address the new needs of the WLAN market. The 802.11a standard uses a frequency of 5 GHz and adopts orthogonal frequency division multiplexing (OFDM) as its modulation scheme. OFDM distributes data among a large number of carriers a predetermined distance apart from one another in a given frequency range. The predetermined distance provides “orthogonality,” which prevents a demodulator from referring to frequencies other than its own frequency.
Related art direct conversion transceivers are a type of transceiver used in wireless systems such as the WLAN. The use of related art direct conversion transceivers is becoming widespread, since they can be implemented in a single-chip structure at a low cost by using a complementary metal oxide semiconductor (CMOS) technology. In addition, related art direct conversion transceiver problems, such as in-phase and quadrature (IQ) mismatch, local oscillator (LO) leakage, and DC offset, can be minimized in a calibration process by using a digital signal processor (DSP) or a microprocessor.
To achieve highly efficient data transmission, a related art transceiver device using a multiple input multiple output (MIMO) method that adopts a plurality of such direct conversion transceivers is being actively developed.
FIG. 1 is a schematic block diagram of a related art MIMO transceiver device 100 that includes first through nth transceivers 110 through 150 having identical structures. The structure of the first transceiver 110 is described below as an example.
The first transceiver 110 includes an up-mixer 111, an envelope detector 112, a down-mixer 113, and a plurality of switches 114 through 116. The up-mixer 111 converts an input baseband signal into an RF signal and outputs the RF signal. The down-mixer 113 converts an input RF signal into a baseband signal and outputs the baseband signal. The envelope detector 112 detects and outputs the envelope of an input signal. The first through third switches 114 through 116 are for forming a plurality of calibration paths. Generally, the envelope detector 112 can be easily implemented using a diode.
Due to their structural characteristics, related art direct conversion transceivers suffer from IQ mismatch, LO leakage, DC offset, and so on. Ideally, the phase difference between an in-phase signal path and a quadrature signal path of a direct conversion transceiver must be 90 degrees, and the gain between the in-phase signal and the quadrature signal must be 0 dB. IQ mismatch occurs when these conditions for the phase difference between the in-phase signal path and the quadrature signal path and the gain are not met. More specifically, I refers to in-phase and Q refers to quadrature. IQ mismatch is caused mainly by imperfect performance of the up-mixer 111 and the down-mixer 113. Also, a major cause of LO leakage is imperfect performance of a local oscillator. Since elements of a direct conversion transceiver on a transmission or receiving path cannot perform ideally, DC offset is present in a baseband signal.
Calibration is required to minimize such related art problems as IQ mismatch. Usually, calibration is performed during the initialization of a transceiver device. A related art calibration process is described below with reference to the MIMO transceiver device 100 of FIG. 1.
Generally, calibration is divided into transmission calibration and receiving calibration. In the transmission calibration, after the first switch 116 and the second switch 114 are turned on and the third switch 115 is turned off, a test signal is transmitted to the up-mixer 111 through a baseband transmission port 117. The up-mixer 111 modulates the test signal to, for example, an RF signal having a high frequency of 5 GHz and outputs the RF signal. The RF signal is demodulated back to the baseband signal by the envelope detector 112 and output through a baseband receiving port 120. The baseband signal output from the baseband receiving port 120 is input to a DSP or a central processing unit (CPU). Based on the baseband signal, the DSP or the CPU carries out calibration of the transceiver device according to a predetermined calibration algorithm.
In the receiving calibration, after the second switch 114 and the third switch 115 are turned on and the first switch 116 is turned off, a test signal is transmitted to the up-mixer 111 through the baseband transmission port 117. The up-mixer 111 modulates the test signal to an RF signal having a high frequency and outputs the RF signal. The RF signal is down-converted back to the baseband signal by the down-mixer 113 and output through the baseband receiving port 120. As in the transmission calibration, the baseband signal output from the baseband receiving port 120 is input to the DSP or the CPU. Based on the baseband signal, the DSP or the CPU carries out calibration according to a predetermined calibration algorithm.
According to the related art, the up-mixer 111 on a transmission path is used in the receiving calibration. Thus, the transmission calibration must precede the receiving calibration. The same calibration process is performed on the second through nth transceivers 130 through 150.
For the calibration of the related art MIMO transceiver device 100, each of the first through nth transceivers 110 through 150 must include an envelope detector. Hence, when the transceiver device 100 is implemented in a single-chip structure, efficiency in the use of space cannot be achieved. In addition, in case of an asymmetric MIMO transceiver device which does not use one of a plurality of transmission paths, a transmission calibration must be performed, even on the non-used transmission path for a receiving calibration, thereby prolonging the overall calibration time.